Scott T. Kay

Revisiting the cosmic cooling crisis

Recent measurements of the K-band luminosity function now provide us with strong, reliable constraints on the fraction of baryons which have cooled. Globally, this fraction is only about 5 per cent, and there is no strong evidence that it is significantly higher in clusters. Without an effective subgrid feedback prescription, the cooled gas fraction in any numerical simulation exceeds these observational constraints, and increases with increasing resolution. This compromises any discussion of galaxy and cluster properties based on results of simulations which include cooling but do not implement an effective feedback mechanism.

The baseline intracluster entropy profile from gravitational structure formation

The radial entropy profile of the hot gas in clusters of galaxies tends to follow a power law in radius outside of the cluster core. Here we present a simple formula giving both the normalization and slope for the power-law entropy profiles of clusters that form in the absence of non-gravitational processes such as radiative cooling and subsequent feedback. It is based on 71 clusters drawn from four separate cosmological simulations, two using smoothed particle hydrodynamics and two using adaptive-mesh refinement (AMR), and can be used as a baseline for assessing the impact of non-gravitational processes on the intracluster medium outside of cluster cores. All the simulations produce clusters with self-similar structure in which the normalization of the entropy profile scales linearly with cluster temperature, and these profiles are in excellent agreement outside of 0.2r 200 . Because the observed entropy profiles of clusters do not scale linearly with temperature, our models confirm that non-gravitational processes are necessary to break the self-similarity seen in the simulations. However, the core entropy levels found by the two codes used here significantly differ, with the AMR code producing nearly twice as much entropy at the centre of a cluster.

The effect of cooling and preheating on the X-ray properties of clusters of galaxies

We calculate X-ray properties of present-day galaxy clusters from hydrodynamical cosmolog- ical simulations of theCDM cosmology and compare these with recent X-ray observations. Results from three simulations are presented, each of which uses the same initial conditions: Non-radiative, a standard adiabatic, non-radiative model; Radiative, a radiative model that includes radiative cooling of the gas; and Preheating, a preheating model that also includes cooling but in addition impulsively heats the gas prior to cluster formation. At the end of the simulations, the global cooled baryon fractions in the latter two runs are 15 and 0.4 per cent, respectively, which bracket the recent result from the K-band luminosity function. We construct cluster catalogues that consist of over 500 clusters and are complete in mass down to 1.18 × 10 13 h −1 M� . While clusters in the Non-radiative simulation behave in accord with the self-similar picture, those of the other two simulations reproduce key aspects of the observed X-ray properties: namely, the core entropy, temperature-mass and luminosity-temperature relations are all in good agreement with recent observations. This agreement stems primarily from an increase in entropy with respect to the Non-radiative clusters. Although the physics affecting the intracluster medium is very different in the latter two models, the resulting cluster entropy profiles are very similar.

Galactic winds in the intergalactic medium

We have performed hydrodynamical simulations to investigate the effects of galactic winds on the high-redshift (z = 3) universe. Strong winds suppress the formation of low-mass galaxies significantly, and the metals carried by them produce C IV absorption lines with properties in reasonable agreement with observations. The winds have little effect on the statistics of the H I-absorption lines, because the hot gas bubbles blown by the winds fill only a small fraction of the volume and because they tend to escape into the voids, thereby leaving the filaments that produce these lines intact. Subject headings: cosmology: observations — cosmology: theory — galaxies: formation — intergalactic medium — quasars: absorption lines

Hydrodynamical simulations of the Sunyaev-Zel'dovich effect: cluster scaling relations and X-ray properties

The Sunyaev–Zeldovich (SZ) effect is a powerful new tool for finding and studying clusters at high redshift, particularly in combination with their X-ray properties. In this paper we quantify the expected scaling relations between these properties using numerical simulations with various models for heating and cooling of the cluster gas. For a Non-radiative model, we find scaling relations in good agreement with self-similar predictions: Y∝T5/2X and Y∝L5/4X. Our main results focus on predictions from Cooling and Preheating simulations, shown recently by Muanwong et al. to provide a good match to the X-ray scaling relations at z= 0. For these runs, we find slopes of approximately Y∝T3X and Y∝LX, steeper and flatter than the self-similar scalings respectively. We also study the redshift evolution of the scaling relations, and find that the slopes show no evidence of evolution out to redshifts well beyond unity, while the normalizations of relations between the SZ signal and X-ray properties do show evolution relative to that expected from self-similarity, particularly at z < 1.

Including star formation and supernova feedback within cosmological simulations of galaxy formation

We investigate phenomenological models of star formation and supernova feedback in N-body/SPH simulations of galaxy formation. First, we compare different prescriptions in the literature for turning cold gas into stars neglecting feedback effects. We find that most prescriptions give broadly similar results: the ratio of cold gas to stars in the final galaxies is primarily controlled by the range of gas densities where star formation is allowed to proceed efficiently. In the absence of feedback, the fraction of gas that cools is much too high, resulting, for example, in a K-band luminosity function that is much brighter than observed. This problem is ameliorated by including a feedback model which either imparts radial kinetic perturbations to galactic gas or directly reheats such material and prevents it from cooling for a certain period of time. In both these models, a significant fraction of cold gas is heated and expelled from haloes with an efficiency that varies inversely with halo circular velocity. Increasing the resolution of a simulation allows a wider dynamic range in mass to be followed, but the average properties of the resolved galaxy population remain largely unaffected. However, as the resolution is increased, more and more gas is reheated by small galaxies; our results suggest that convergence requires the full mass range of galaxies to be resolved.

Cosmological simulations of the intracluster medium

We investigate the properties of the intracluster medium (ICM) that forms within N-body/hydrodynamical simulations of galaxy clusters in aCDM cosmology. When ra- diative cooling and a simple model for galactic feedback are included, our clusters have X-ray luminosities and temperatures in good agreement with observed systems, demonstrating the required excess entropy in their cores. More generally, cooling and feedback increases the entropy of the ICM everywhere, albeit without significantly affecting the slope of the pro- file (S ∝ r )a tlarge radii. The temperature of the ICM is only modestly increased by these processes, with projected temperature profiles being in reasonable agreement with the obser- vations. Star/galaxy formation is still too efficient in our simulations, however, and so our gas mass fractions are around 60 per cent of the observed value at r2500. Finally, we examine the reliability of using the hydrostatic equilibrium equation to estimate cluster masses and find that it underpredicts the true mass of our clusters by up to 20 per cent, due to incomplete thermalization of the gas. Feedback reduces this discrepancy, however, with estimates being accurate to within 10 per cent out to r500. Ke yw ords: hydrodynamics - methods: numerical - X-rays: galaxies: clusters.

THE EFFECT OF RADIATIVE COOLING ON SCALING LAWS OF X-RAY GROUPS AND CLUSTERS

We have performed cosmological simulations in a ΛCDM cosmology with and without radiative cooling in order to study the effect of cooling on the cluster scaling laws. Our simulations consist of 4.1 million particles each of gas and dark matter within a box size of 100 h-1 Mpc, and the run with cooling is the largest of its kind to have been evolved to z = 0. Our cluster catalogs both consist of over 400 objects and are complete in mass down to ~1013 h-1 M☉. We contrast the emission-weighted temperature-mass (Tew-M) and bolometric luminosity-temperature (Lbol-Tew) relations for the simulations at z = 0. We find that radiative cooling increases the temperature of intracluster gas and decreases its total luminosity, in agreement with the results of Pearce et al. Furthermore, the temperature dependence of these effects flattens the slope of the Tew-M relation and steepens the slope of the Lbol-Tew relation. Inclusion of radiative cooling in the simulations is sufficient to reproduce the observed X-ray scaling relations without requiring excessive nongravitational energy injection.

Entropy amplification from energy feedback in simulated galaxy groups and clusters

We use hydrodynamical simulations of galaxy clusters and groups to study the effect of pre–heating on the entropy structure of the intra–cluster medium (ICM). Our simulations account for non–gravitational heating of the gas either by imposing a minimum entropy floor at redshift zh = 3 in adiabatic simulations, or by considering feedback by galactic winds powered by supernova (SN) energy in runs that include radiative cooling and star formation. In the adiabatic simulations we find that the entropy is increased out to the external regions of the simulated halos as a consequence of the transition from clumpy to smooth accretion induced by extra heating. This result is in line with the predictions of the semi–analytical model by Voit et al. However, the introduction of radiative cooling substantially reduces this entropy amplification effect. While we find that galactic winds of increasing strength are effective in regulating star formation, they have a negligible effect on the entropy profile of cluster–sized halos. Only in models where the action of the winds is complemented with diffuse heating corresponding to a pre–collapse entropy do we find a sizable entropy amplification out to the virial radius of the groups. Observational evidence for entropy amplification in the outskirts of galaxy clusters and groups therefore favours a scenario for feedback that distributes heating energy in a more diffuse way than predicted by the model for galactic winds from SN explosions explored here.

The evolution of clusters in the CLEF cosmological simulation: X-ray structural and scaling properties

We present results from a study of the X-ray cluster population that forms within the CLEF cosmological hydrodynamics simulation, a large N-body/SPH simulation of the Lambda cold dark matter cosmology with radiative cooling, star formation and feedback. With nearly 100 (kT > 2 keV) clusters at z = 0 and 60 at z = 1, our sample is one of the largest ever drawn from a single simulation and allows us to study variations within the X-ray cluster population both at low and high redshift. The scaled projected temperature and entropy profiles at z = 0 are in good agreement with recent high-quality observations of cool core clusters, suggesting that the simulation grossly follows the processes that structure the intracluster medium (ICM) in these objects. Cool cores are a ubiquitous phenomenon in the simulation at low and high redshift, regardless of a cluster’s dynamical state. This is at odds with the observations and so suggests there is still a heating mechanism missing from the simulation. The fraction of irregular (major merger) systems, based on an observable measure of substructure within X-ray surface brightness maps, increases with redshift, but always constitutes a minority population within the simulation. Using a simple, observable measure of the concentration of the ICM, which correlates with the apparent mass deposition rate in the cluster core, we find a large dispersion within regular clusters at low redshift, but this diminishes at higher redshift, where strong cooling-flow systems are absent in our simulation. Consequently, our results predict that the normalization and scatter of the luminosity‐temperature relation should decrease with redshift; if such behaviour turns out to be a correct representation of X-ray cluster evolution, it will have significant consequences for the number of clusters found at high redshift in X-ray flux-limited surveys.